The principle of using laser optics to induce a phase-change phenomenon in a chalcogenide thin film has been well established. By irradiating the film with the appropriate laser condition, the film can be reversibly converted between crystalline and amorphous phases. Such films are referred to as phase-change materials or films. Various laser beam optical systems have been devised to write or erase information by altering the phase of the film and to read digitized information by means of different optical properties (extinction or scattering) between the amorphous and crystalline phases of the film. Both far-field and near-field laser beam optical systems and techniques have been developed. Many workers have optimized the cyclability, stability and optical performance of such films and layered them within an optical media stack on a disk substrate to form structures for either reflective or transmissive optical writing and reading. Such information storage products are commonly referred to as phase-change optical storage media. The method of using a laser beam system to record information on phase-change optical storage media (e.g. disks and tapes) has been commonly referred to as phase-change optical recording.
Most of the phase-change optical media developed for use in the prior art were designed for far-field optical devices wherein the active phase-change film is imbedded in a relatively thick optical stack (on the order of a few .mu.m) on a base substrate for either reflective or transmissive read/write optical methods. Such optical media structures often include: (1) a base substrate, (2) a dielectric layer with low thermal conductivity and high index of refraction, (3) the phase-change film layer, (4) a second dielectric layer with low thermal conductivity and high index of refraction, (5) one or more reflective layers, and (6) a protective polymer resin over-coat. Optical media structures exemplary of the prior art are disclosed in Akahira, et al., U.S. Pat. No. 5,527,661; Oakda, et al., U.S. Pat. No. 5,521,901; Kawahara, et al., U.S. Pat. No. 5,453,346; Yamada, et al., U.S. Pat. No. 5,424,106; and Yamada, et al., 5,545,454.
In the prior art, the phase-change recording layer is typically disposed between the two dielectric layers. The dielectric layers provide heat resistance to protect thermally sensitive materials, such as polycarbonate substrates and resin over-coats, prevent deformation or evaporation of the phase-change film, transmit the laser beam energy efficiently to the active phase-change layer and provide proven mechanical stability through thermal cycling of the phase-change media. Because of its low thermal conductivity, high index of refraction, and non-reactivity with phase-change materials, ZnS--SiO.sub.2 is often used as the preferred dielectric and is often applied by sputter deposition techniques. Yamada, et al., U.S. Pat. No. 5,545,454, suggested that several other dielectric compounds may also be used including SiO.sub.2, ZrO.sub.2, TiO.sub.2, Ta.sub.2 O.sub.5, BN, Si.sub.3 N.sub.4, AlN, TiN, ZnS, PbS, SiC, diamond and diamond-like carbon. These prior art media structures are intended for application within far-field optical devices. Realizable phase-change recording systems with far-field optics devices have just recently reached areal densities of 300 Mbits/cm.sup.2 and, with the proposed use of blue-laser technology, are expected to approach 1 gigabit per square centimeter (Gbit/cm.sup.2) densities. (See N. Yamada, Erasable Phase-Change Optical Materials, MRS Bulletin, Vol. (9), September 1996, pages 48-50.)
Much higher areal densities are possible with near-field optical devices. Betzig, et al. (Applied Physics Letters, Vol. 61(2), Jul. 13, 1992, pages 142-143) demonstrated that a near-field optical probe positioned at 100 .ANG. above the surface of a magneto-optic recording film could produce .about.600 .ANG. bit sizes and resolve images of .about.300-500 .ANG. in size. Their work suggests that near-field optics are capable of writing/reading information on optically active media, including phase-change optical media, so as to obtain areal densities as high as .about.16 Gbits/cm.sup.2 (or .about.100 Gbits/in.sup.2). The areal density of bit information that can be achieved by near-field optics is determined by the smallest reproducible bit size of d.sub.s, the size of the probe beam d.sub.p from the near-field optical device, and the optical distance between the probe and phase-change optical recording layer h. It is desirable to have h about less than about d.sub.p /2 in order to produce a domain size d.sub.s about the size d.sub.p. For example, if it is desirable to obtain a 500 .ANG. bit size (for a practical areal density of .about.16 Gbits/cm.sup.2) by means of a near-field laser beam with d.sub.p .apprxeq.500 .ANG., then it is necessary to have h.apprxeq.250 .ANG. or less.
Near-field optical devices have been developed wherein a semiconductor laser is coupled closely to the surface of a phase-change optical storage medium. Exemplary of the prior art disclosing near-field optical devices used for phase-change media are Ukita, et al., U.S. Pat. No. 4,860,276 and Hopkins, et al., U.S. Pat. No. 5,625,617. In such near-field optical devices, a sub-wavelength semiconductor laser or laser array may be integrated into a "slider" mechanism, typically of aerodynamic design, which allows the laser system to be held over and close to the surface of the spinning medium. The slider containing the optical head may be referred to as an "optical-head-slider." In order to realize commercially viable areal densities (i.e. substantially greater than 2 Gbits/cm.sup.2) with such near-field optic devices, the value of h should be no more than 1250 .ANG.. It is preferable that this distance be near or less than 100 .ANG. to obtain the highest possible areal densities.
Sliders with magnetic recording heads (i.e. a magnetic-head-slider) and related air bearing mechanism and methods are widely known and used in magnetic memory recording systems. It is also well known that components used in magnetic recording media systems (e.g. magnetic transducers, magnetic-head-sliders and thin film alloy disks) are susceptible to wear and corrosion to damage as the air bearing surface of the slider contacts the memory storage disk during start-up and shut down of the disk rotation and during operation. Protective diamond-like carbon over-coats are often applied to components to protect them from corrosion and wear, to allow good wetting and reflow of topical lubricants, and to improve the life time of the data storage recording system and media. It would be desirable if similar protective diamond-like carbon over-coat technology were available for phase-change optical recording media to enable the use of near-field optic systems at low flight heights between the optical-head-slider and the media surface.
Amorphous diamond-like carbon (DLC) films are so-named because their properties resemble, but do not duplicate, those of diamond. Some of these properties are high hardness (about 3 to about 22 GPa), low friction coefficient (approximately 0.1) and transparency across the majority of the electromagnetic spectrum. At least some of the carbon atoms in DLC are bonded in chemical structures similar to that of diamond, but without long range crystal order. Although the term DLC was initially intended to define a pure carbon material, the term DLC now includes amorphous, hard carbon materials containing up to 50 atomic percent of hydrogen. Other names for these hydrogen-containing DLC materials are "amorphous hydrogenated carbon", hydrogenated diamond-like carbon, or diamond-like hydrocarbon. The structure of these hydrogen-containing hard carbon materials may be best described as a random covalent network of graphitic-type structures interconnected by sp.sup.3 linkages, although the definitive structure of the films has yet to be universally accepted. In addition to the use of carbon and hydrogen, DLC may be doped with other elements or combination of elements. The addition of such elements, e.g. silicon and germanium, can provide or enhance useful material properties such as wear resistance, adhesion, hardness, stress, and oxidation resistance. The term "DLC" is used in the discussion of the present invention to refer to both the amorphous non-hydrogenated hard carbon materials, amorphous hydrogenated hard carbon materials and doped modifications of these two materials.
Many methods for directly depositing DLC films are known in the prior art, including (i) direct ion beam deposition, dual ion beam deposition, glow discharge, radio frequency (RF) plasma, direct current (DC) plasma or microwave plasma deposition from a carbon-containing gas or vapor which can also be mixed with hydrogen and/or inert gas and/or other gases containing doping elements, (ii) electron beam evaporation, ion-assisted evaporation, magnetron sputtering, ion beam sputtering, or ion-assisted sputter deposition from a solid carbon or doped carbon target material, or (iii) combinations of (i) and (ii).
DLC films are well known in the art and have been recognized as potential coatings to enhance the abrasion resistance of various substrate materials, including recording media as discussed above. The DLC coatings possess excellent mechanical properties such as high hardness and low coefficient of friction, and exhibit excellent resistance to abrasion and chemical attack by nearly all known solvents, bases, and acids. However, it has been found that the DLC coatings will impart improved wear resistance to the substrate only if the adherence of the coating to the parent substrate is excellent.
The most obvious and common approach to coating a substrate is to apply the DLC coating directly onto a clean surface which is free of residue. However, this approach often results in a DLC coating which displays inadequate adhesion, and therefore, poor wear resistance. DLC coatings are typically under significant compressive stress, on the order of 0.5 to approximately 5 GPa. This stress greatly affects the ability of the coating to remain adherent to the substrate. Additionally, the surface of the substrate to be coated often contains alkali metals, oxides, and other contaminants which can inhibit bonding of the DLC coating. Therefore, novel and non-obvious methods are often required to produce a particular substrate with a highly adherent DLC coating which provides excellent abrasion resistance.
As noted in U.S. Pat. Nos. 5,545,454 and 5,424,106, Yamada, et al. suggest that DLC materials of unspecified composition could be used in the dielectric layers of the phase-change media structure. However, the DLC layers as discussed by Yamada, et al. would act as optical dielectric layers with their thickness predetermined by an optical performance criterion and calculations, and would not be intended to act as a tribologically and environmentally protective over-coat. Moreover, the phase-change media structures proposed in this and other prior art examples are not intended for use with near-field optic systems.
Protective non-doped DLC over-coats or hydrogenated DLC over-coats are widely used in the magnetic recording media industry. Exemplary of the prior art are Michihide, et al., EP 216 079 A1; Howard, U.S. Pat. No. 4,778,582; Meyerson, et al., U.S. Pat. No. 4,647,494; Japanese Laid Open Pat. Application (Kokai) No. 1-287819, Shinohara; Endo, et al. U.S. Pat. No. 4,774,130; Kurokawa. U.S. Pat. No. 4,717,622; and Nakamura, et al., U.S. Pat. No. 4,804,590. It has been widely established that protective DLC over-coatings produced by prior art means with magnetron sputtering, chemical vapor deposition, plasma chemical vapor deposition, plasma chemical vapor deposition, or plasma-injected chemical vapor deposition, can be no less than about 100 .ANG. thick. The predominant production method for depositing DLC over-coats onto magnetic media is magnetron sputtering. Below this 100 .ANG. threshold, protective performance of DLC coatings produced by magnetron sputtering on magnetic media disks becomes unacceptable based on conventional contact stop-start (CSS) tests of 20,000 to 50,000 cycles. Currently the magnetic media storage industry is looking for an alternative means to magnetron sputtering which can produce a DLC over-coat with satisfactory environmental and tribological performance at a thickness below 100 .ANG. in order to further increase areal density.
An alternative method of forming a DLC over-coat is by direct ion beam deposition as described by Knapp, et al. in International Application under the PCT, WO 95/23878, published Sep. 8, 1995. It has been demonstrated that protective coatings produced by direct ion beam deposition provide superior environmental and tribological performance using established production methods for DLC over-coat thicknesses of about 50 .ANG. on magnetic recording media. It is believed that such DLC coatings are superior to DLC coatings produced by other deposition techniques in that ion beam-deposited DLC has better morphology at such low film thicknesses. This can be attributed to the favorable surface energetics which are more readily selected and controlled by the direct ion beam deposition process.
Even with the successful performance of direct ion beam deposition of DLC on magnetic recording media, sliders and heads, it is not evident from the prior art how well ion beam deposited DLC films will perform on the surface of phase-change media. To the knowledge of the inventors, there is no prior art or teaching of protective DLC over-coats on phase-change media structures. Also, it is not obvious to one of ordinary knowledge and skill in the art based on the prior art how to adhere thin DLC coatings to either a phase-change recording layer or related dielectric surface layers so as to enable the use of near-field optical-head-sliders without damage to the phase-change media. A wide variety of materials have been used for adhesion-promotion of DLC over magnetic media and devices. Some of these adhesion-promoting materials include the following: amorphous silicon, silicon carbide, silicon nitride, silicon oxide, silicon oxy-nitride, and mixtures thereof (in some cases containing hydrogen), chrome, titanium, and germanium. It is not clear what material, if any will enhance adhesion of the DLC over-coats to standard dielectric surfaces, e.g. ZnS--SiO.sub.2, or to phase-change recording layers. It is preferable to have a DLC over-coat that adheres well to the phase-change recording layer directly without the need for either adhesion or dielectric layers. Materials that have been suggested for such recording layers include, for example, alloys such as Ge--Sb--Te, In--Sb--Te, Sb--Te, Ge--Sb--Te--Pd, Ag--Sb--In--Te, Ge--Bi Sb--Te, Ge--Bi--Te, Ge--Sn--Te, Ge--Sb--Te--Se, Ge--Bi--Tc--Si, and Ge--Te--Sn--Au; with Ge.sub.2 Sb.sub.2 Te.sub.5 shown to have specific application as a recording layer; see Yamada, et al., U.S. Pat. No. 5,545,454.
As discussed earlier, it is desirable to minimize the optical thickness of a DLC over-coat and related layers in order to achieve the highest possible areal densities. The thickness and index of refraction .eta. of the DLC over-coat and all other additional sub-layers deposited on top of the phase-change optical recording layer determine the optical path length h between the near-field optic and the recording layer. The index of refraction for DLC and dielectrics like ZnS--SiO.sub.2 can be as high as 2. Thus, their optical thickness is nearly double their physical thickness. Contemporary slider technology allows read/write devices to fly at about 250 .ANG. over a conventional DLC over-coat on magnetic media without damage to the medium surface. It is reasonable to expect that a similar flight distance for a near-field read/write optical device on a slider is possible for DLC over-coats on a phase-change optical structure. Therefore, with an upper limit of h being about 1250 .ANG. for viable areal densities in a phase-change optical medium, a DLC over-coat should be no more than 450 .ANG. and preferably about 50 .ANG. to allow for other thin, e.g., about 10 .ANG. to about 100 .ANG., dielectric layers and adhesion-promoting interlayers if needed.
It would be desirable for such a DLC over-coat for phase-change media devices to have the following environmental protection and tribological properties:
(i) low friction and excellent wear-resistance performance to enable the slider and head to fly by means of the air bearing mechanism over the media; PA1 (ii) high hardness (&gt;8 GPa); PA1 (iii) small overall thickness that is substantially less than 450 .ANG. to enable high data storage densities with near-field laser optics; PA1 (iv) excellent adhesion to the surface of the phase-change media structure with a thin interlayer or with no interlayer; PA1 (v) stable material properties with respect to diffusion and interaction with the underlying phase-change media structure; PA1 (vi) favorable wetting and interaction with topical lubricants if used; and PA1 (vii) excellent corrosion protection properties. Furthermore the process by which to produce such an over-coat should allow for high deposition rates (i.e. greater than 10 .ANG./sec) of the DLC layer and be integratable into manufactured platforms for mass production of near-field phase-change optical recording media devices.